Pre-press color-proofing is a procedure that is used by the printing industry for creating representative images of printed material without the high cost and time that is required to produce printing plates and set up a high-speed, high volume, printing press to produce single proof of an intended image. One such commercially available image processing apparatus, described in commonly assigned U.S. Pat. No. 5,268,708, has half-tone color proofing capabilities. This image processing apparatus forms an intended image on a sheet of thermal print media by transferring dye from a sheet of dye donor material to the thermal print media by thermal energy to the dye donor sheet material. This image processing apparatus is comprised generally of a material supply assembly or carousel, lathe bed scanning subsystem (which includes a lathe bed scanning frame, translation drive, translation stage member, printhead, and vacuum imaging drum), and thermal print media and dye donor sheet material exit transports.
The operation of the image processing apparatus comprises metering a length of the thermal print media (in roll form) from the material assembly carousel. The thermal print media is then measured and cut into sheet form of the required length and transported to the vacuum imaging drum, registered, and wrapped around and secured on the vacuum imaging drum. Next a length of dye donor material (in roll form) is metered from the material supply assembly carousel, measured, and cut into a sheet of the required length. The sheet of donor material is transported to and wrapped around the vacuum imaging drum, superposed in registration with the thermal print media.
After the dye donor sheet material is secured to the periphery of the vacuum imaging drum, the vacuum imaging drum with thermal print media and dye donor sheet material attached, is rotated at a constant speed. A translation drive then traverses the printhead and translation stage member axially along the vacuum imaging drum in coordinated motion with the rotating vacuum imaging drum. These movements combine to produce the intended image on the thermal print media.
After the intended image has been written on the thermal print media, the dye donor sheet material is then removed from the vacuum imaging drum. This is done without disturbing the thermal print media that is beneath it. The dye donor sheet material is then transported out of the image processing apparatus by the dye donor sheet material exit transport. Additional sheets of dye donor sheet material, each sheet a different color, are sequentially superimposed with the thermal print media on the vacuum imaging drum and imaged onto the thermal print media until the intended image is completed. The completed image on the thermal print media is then unloaded from the vacuum imaging drum and transported to an external holding tray on the image processing apparatus by the receiver sheet material exit transport.
The scanning subsystem comprises the mechanism that provides the mechanical actuators for the vacuum imaging drum positioning and motion control to facilitate placement, loading onto, and removal of the thermal print media and the dye donor sheet material from the vacuum imaging drum. The scanning subsystem provides the scanning function by retaining the thermal print media and dye donor sheet material on the rotating vacuum imaging drum. The scanning subsystem generates a once per revolution timing signal to the data path electronics as a clock signal, while the translation drive traverses the translation stage member and printhead axially along the vacuum imaging drum in a coordinated motion, while the vacuum imaging drum rotates past the printhead to allow precise control of the placement of each pixel, in order to produce the intended image on the thermal print media.
The lathe bed scanning frame provides the structure to support the vacuum imaging drum and its rotational drive. The translation drive with the translation stage member and printhead are supported by the two translation bearing rods that are substantially straight along their longitudinal axis and are positioned parallel to the vacuum imaging drum and lead screw. Consequently, they are parallel to each other therein forming a plane with the vacuum imaging drum and lead screw. The translation bearing rods are, in turn, supported by the outside walls of the lathe bed scanning frame of the lathe bed scanning subsystem or write engine. The translation bearing rods are positioned and aligned, permitting low friction movement of the translation stage member and the translation drive. The translation bearing rods are rigid, and prevent sag or distort between the mounting points at their ends. They are arranged parallel with the axis of the vacuum imaging drum. The front translation bearing rod is arranged to locate the axis of the printhead precisely on the axis of the vacuum imaging drum with the axis of the printhead located perpendicular, vertical, and horizontal to the axis of the vacuum imaging drum. The translation stage member, front bearing is arranged to form an inverted "V" and provides only that constraint to the translation stage member. The translation stage member with the printhead mounted on the translation stage member, is held in place by only it's own weight. The rear translation bearing rod locates the translation stage member with respect to rotation of the translation stage member about the axis of the front translation bearing rod. This is done to prevent binding, chatter, or otherwise undesirable vibration or jitter of the translation drive or printhead during the writing process, which would cause an unacceptable artifacts in the intended image. This is accomplished by the rear bearing which engages the rear translation bearing rod only on diametrically opposite side of the translation bearing rod on a line perpendicular to a line connecting the centerlines of the front and rear translation bearing rods.
The translation drive provides relative movement of the printhead by means of a DC servo motor and encoder which rotates a lead screw parallel with the axis of the vacuum imaging drum. The printhead is placed on a translation stage member in "V" shaped grooves, which are formed in the translation stage member, and which are in a precise relationship to the bearings for the front translation stage member supported by the front and rear translation bearing rods. These translation bearing rods are positioned parallel to the vacuum imaging drum, so that it automatically adopts the preferred orientation with respect to the vacuum imaging drum. The printhead is selectively locatable with respect to the translation stage member, and is thus positioned with respect to the vacuum imaging drum surface. By adjusting the distance between the printhead and the vacuum imaging drum surface, as well as angular position of the printhead about it's axis, an accurate means of adjustment for the printhead is provided. An extension spring provides a load against these two adjustment means.
The translation stage member and printhead are attached to a rotatable lead screw (having a threaded shaft) by a drive nut and coupling. The coupling is arranged to accommodate misalignment of the drive nut and lead screw so that only forces parallel to the linear lead screw and rotational forces are imparted to the translation stage member by the lead screw and drive nut. The lead screw rests between two sides of the lathe bed scanning frame of the lathe bed scanning subsystem or write engine, where it is supported by deep groove radial bearings. At the drive end the lead screw continues through the deep groove radial bearing, through a pair of spring retainers, that are separated and loaded by a compression spring to provide axial loading, and to a DC servo drive motor and encoder. The DC servo drive motor induces rotation to the lead screw moving the translation stage member and printhead along the threaded shaft as the lead screw is rotated. The lateral directional movement of the printhead is controlled by switching the direction of rotation of the DC servo drive motor and thus the lead screw.
An autofocus system uses reflected light energy from the thermal print media surface to actively correct for variations in distance between the printhead and the dye donor during the scanning process. An additional light source is used for this focusing application. The light source has a wavelength of approximately 960 nanometers. With this wavelength, the dye layer of the dye donor sheet is essentially transparent and reflected light from the surface of the thermal print media is re-imaged in the plane of a dual cell silicon focus detector arrangement. An analog closed loop servo uses the focus detector error signal to drive an electromagnetic actuator, which moves the last lens element of a lens system to correct for dye layer variability. The servo has both a proportional path and an integral path. In normal operation, the servo drives the focusing system to a near-zero focus error signal. If the position of the last lens element with the closed loop focus error signal at zero, dose not yield a focus position for the maximum transfer of dye to the thermal print media, an offset is applied to the closed loop to operate at a position other than the one that corresponds to the near-zero focus error signal. In this application the circumference of the vacuum imaging drum is larger than the thermal print media and the dye donor along with an axially extending flat that causes the focus system to re-establish the focus position at the beginning of the thermal print media with every revolution of the vacuum imaging drum due to the discontinuation of the tracking surface.
The vacuum imaging drum is cylindrical in shape and includes a hollowed-out interior portion, and further includes a plurality of holes extending through its housing for permitting a vacuum to be applied from the interior of the vacuum imaging drum for supporting and maintaining the position of the thermal print media and dye donor sheet material as the vacuum imaging drum rotates. The ends of the vacuum imaging drum are enclosed by cylindrical plates. The cylindrical end plates are each provided with a centrally disposed spindle which extends outwardly through support bearings and are supported by the lathe bed scanning frame. The drive end spindle extends through the support bearing and is stepped down to receive a DC drive motor armature which is held on by means of a nut. A DC motor stator is stationarily held by the lathe bed scanning frame member, encircling the armature to form a reversible, variable speed DC drive motor for the vacuum imaging drum. An encoder is mounted at the end of the spindle to provide timing signals to the image processing apparatus. The opposite spindle is provided with a central vacuum opening, which is in alignment with a vacuum fitting with an external flange that is rigidly mounted to the lathe bed scanning frame. The vacuum fitting has an extension which extends within but is closely spaced from the vacuum spindle, thus forming a small clearance. With both the thermal print media and dye donor sheet material completely loaded on the vacuum imaging drum the internal vacuum level of the vacuum imaging drum is approximately 50-60 inches of water in this configuration.
The outer surface of the vacuum imaging drum is provided with an axially extending flat, which extends over approximately eight degrees of the vacuum imaging drum circumference. The vacuum imaging drum is also provided with a circumferential recess which extends circumferentially from one side of the axially extending flat circumferentially around the vacuum imaging drum to the other side of the axially extending flat, and from approximately one inch from one end of the vacuum imaging drum, to approximately one inch from the other end of the vacuum imaging drum. The thermal print media when mounted on the vacuum imaging drum is seated in the circumferential recess and therefor the circumferential recess has a depth substantially equal to the thermal print media thickness seated there within which is approximately 0.004 inches in thickness. The purpose of the circumferential recess on the vacuum imaging drum surface is to eliminate any creases in the sheets of the dye donor sheet material, as they are drawn over the thermal print media during the loading of the dye donor sheet materials. This assures that no folds or creases will be generated in the dye donor sheet materials which could extend into the image area and seriously adversely affect the intended image. The circumferential recess also substantially eliminates the entrapment of air along the edge of the thermal print media, where it is difficult for the vacuum holes in the vacuum imaging drum surface to assure the removal of the entrapped air. Any residual air between the thermal print media and the dye donor sheet material, can also adversely affect the intended image.
The purpose of the extending flat is two-fold. First, it assures that the leading and trailing ends of the dye donor sheet material are protected from the effect of the air during the relatively high speed rotation that the vacuum imaging drum undergoes during the imaging process. Thus the air will have less tendency to lift the leading or trailing edges of the dye donor sheet material. The vacuum imaging drum axially extending flat also ensures that the leading and trailing ends of the dye donor sheet material are recessed from the vacuum imaging drum periphery. This reduces the chance that the dye donor sheet material can not come in contact with other parts of the image processing apparatus, such as the printhead, causing a jam and possible loss of the intended image or worse, catastrophic damage to the image processing apparatus.
The vacuum imaging drum axially extending flat also acts to impart a bending force to the ends of the dye donor sheet materials when they are held onto the vacuum imaging drum surface by vacuum from within the interior of the vacuum imaging drum. Consequently when the vacuum is turned off to that portion of the vacuum imaging drum, the end of the dye donor sheet material will tend to lift from the surface of the vacuum imaging drum. Thus turning off the vacuum eliminates the bending force on the dye donor sheet material, and is used as an advantage in the removal of the dye donor sheet material from the vacuum imaging drum.
Although the image processing apparatus described above is satisfactory, it is not without room for improvement. Image quality specifications for existing image processing apparatus requires scanning subsystem tolerance in the 10 micron range. This tolerance is directly related to variations in the distance between the printhead and the dye donor material, which in turn, is related to the surface variations of the vacuum imaging drum, and movement of the linear translation system. To correct for this variability, a focusing system is required, adding mechanical complexity and increasing cost. It is desirable to minimize these variations which would result in improvement of the image quality and eliminate the need for a focusing system, reducing the mechanical complexity and decreasing the cost of the image processing apparatus.
Prior art systems of finishing the surface of a drum move a cutting or grinding tool into contact with the surface of the rotating drum and translate the tool in an axial direction from a first end of the drum to a second end of the drum. When the cutting tool reaches the second end of the drum, it is retracted, translated axially to the starting position, and again moved into contact with the surface of the drum at a position closer to the axis of rotation of the drum. This method of finishing the surface of the drum, however, has some drawbacks in that the pressure of the tool against the surface of the rotating drum builds mechanical stress into the finishing tool, making it difficult to position the cutting tool the correct incremental distance closer to the axis of rotation similar to a historesis effect. Also, rotating drums are conventionally finished and then assembled into a completed apparatus, such as an image processing apparatus. Thus, regardless of the degree of surface finish, other variations in the completed apparatus, such as wear of the bearings, variations in lead screw tolerances, and translation movements of the printhead, add additional cumulative errors.